As is well known, a heat pump usually comprises a compressor, two coils, an expansion device and a reversing valve. The compressor compresses refrigerant into one of the coils which consequently operates as a condenser coil and thereafter the refrigerant liquid passes through an expansion valve and into the other coil, operating as an evaporator. Refrigerant vapor leaves the evaporator to return to the inlet of the compressor, normally through an accumulator. A four-way reversing valve may be utilized to direct the refrigerant from the compressor into either of the two coils and simultaneously direct the output from the other coil to the input of the compressor. In this manner the "role" of the coils can be reversed. The coil operating as a condenser releases heat, normally to air passing over it, while the coil operating as an evaporator will absorb heat, normally from air passing over it. Usually, when one coil is mounted inside and the other outside of a building, the heat pump is considered to be in a heating mode or phase when the inside coil acts as a condenser and in the cooling mode or phase when the inside coil acts as an evaporator.
A problem occurs when the reversal valve of such a heat pump is switched from one mode to the other in that, at the time that the mode is switched, the coil acting as a condenser, and therefore containing refrigerant under high pressure, becomes connected to the compressor intake, while the coil acting as an evaporator, and therefore containing refrigerant at a low pressure, is connected to the compressor output. The result is high pressure refrigerant discharging through the compressor, often still in a liquid state. Aside the objectionable noise caused by refrigerant rushing through the compressor, there is a possibility of damage to the compressor valves. Additionally, dilution of the compressor lubricant by liquid refrigerant will accelerate the deterioration of the compressor due to inadequate lubrication.
A number of solutions to this problem have been suggested by the prior art. For example, U.S. Pat. No. 3,230,728 to Biehn teaches the use of a capillary shunt to the line leading to the input of the compressor. A valve blocks the main line returning to the compressor, forcing the refrigerant to flow through the capillary, for a period of time sufficient to allow the pressure to equalize in the system. U.S. Pat. No. 3,128,607 to Kyle shows a device which provides a time delay on the reversal valve which, when the heating thermostat is satisfied after a period of indoor air heating and although the compressor motor is deenergized, maintains the reversal valve in its energized "heating" mode position for a predetermined period of time rather than allowing it to return to its non-energized "cooling" mode position.
Although it is known to provide a time delay before restarting the compressor of a non-reversible refrigeration unit after the compressor has been deactivated, this is a non-analogous situation to that encountered with respect to the reversal of phase in a heat pump. As noted above, the problems encountered with a reversal in phase of a heat pump are caused by the simultaneous connection of the compressor inlet to a high pressure coil and the compressor outlet to a low pressure coil allowing refrigerant to rush through the compressor. The compressor in a refrigeration unit, on the other hand does not encounter such a reversal, rather, after running for a while and when restarting it is subjected to a high pressure coil connected to its outlet and a low pressure coil connected to its inlet. This places a loud on the motor which usually causes it to stall, overloading the motor and possibly damaging the unit.